Are you thinking to get rid of something and want to make them “unfriend” in “facebook”? Think twice. Because once you make them unfriend you can’t block them at all. After making “unfriend” their stories only remove from your “news feed” page. Anytime if they want they can visit your page and see what you keep as public and you may also go and visit their pages. Both you and them can comment, like or share any story of your or them. And you can’t stop it no matter how you want later, because you can’t block each other.
Another thing, once you “block” any facebook id, both you and they won’t see each other’s movement. In any weak moment, you might want to see what are they doing, you can unblock them, and can see what is happening. Then what? No you can’t block them now. Now both of you will be like “unfriend”, only each other’s news won’t show on your news feed.
Same situation you will face in Yahoo messenger too.